The production of polyvinyl chloride by emulsion, suspension and homogenization polymerization in the aqueous phase is well-known. Likewise conventional is the array of problems encountered with respect to wall deposits in conducting these processes on a commercial scale. Coatings of polyvinyl chloride are formed as paper-like layers or crusts on the inner wall of the reactor and on the units installed within the reactor. As is known, this formation of coatings and crusts not only reduces the polymer yield and the cooling capacity of the polymerization reactor, but also impairs the product quality, since the flakes of polymer become detached from the reactor walls and mix with the product. Therefore, it is necessary to open and clean the reactor after each polymerization cycle or after only a few polymerization cycles. Due to the time-consuming and difficult removal of the polymer coating from the reactor walls, which requires a great amount of work, the capacity of a polymerization reactor is reduced and a continuous polymerization is impossible.
Many attempts have been made to prevent such polymer deposit formations on the reactor walls. However, a satisfactory solution of the problem has not been attained heretofore.
It is known that alkaline-reacting compounds can be added to a polymerization system to reduce the deposition of flakes of polymer on the walls of the reactor [German Unexamined Laid-Open Application] DOS 2,208,796. According to DOS 1,745,391, to decrease the polymer deposits, the polymerization of vinyl chloride is carried out in the presence of a dispersing agent, a nonionic wetting agent and an inert solvent in an aqueous medium which is maintained alkaline. According to these teachings, an alkaline pH value must be strictly adhered to, if cakings are to be reduced.
DOS 2,044,259 describes, as a measure for preventing the formation of coatings, the application of at least one polar organic compound and/or at least one organic dye and/or at least one inorganic pigment as such or in a dissolved and/or diluted form to the surface of the internal walls of the reactor, to the agitator vanes, the baffles, the cooler and the conduits connecting the cooler with the polymerization reactor. A further reduction of the formation of polymer deposits is attained by adding to the polymerization charge a quantity of an alkali or alkaline earth hydroxide or oxide of such a quantity that the pH of the charge, after the polymerization has been completed, is at least 6, preferably at least 8. If the walls of the polymerization reactor are metal, a further improved effect is attained by adding an organic oxide to the mother liquor. If, in the aforementiond process, an oxidizing agent is also added to the aqueous phase prior to polymerization, the effectiveness of the above-mentioned oxides is enhanced.
According to DOS 2,212,962, the formation of deposits is prevented by the use of rapidly acting catalysts while maintaining an acidic pH value and operating with a comparatively cool reactor wall. Although the hydrogen chloride produced during the polymerization is not neutralized in this procedure, the reduction in polymerization velocity, which occurs in the case of a more strongly alkaline medium, is avoided and the graft polymerization of the monomers to the protective colloid, considered an absolute necessity in this method, is not suppressed.
According to DOS 1,645,397, a reduction in the deposit formation can be achieved by homogenizing the aqueous dispersion which contains protective colloids, polymerizing the charge under agitation, stopping the agitation at a specific conversion, and later on continuing the process again under agitation.
The measures as taken above, such as the addition of special compounds to the polymerization charge, new polymerization recipes, preparing the inner surfaces of the reactor with dyes, as well as methods of process technology, e.g., as in DOS 1,645,397, by interruption of agitation, are expensive and/or of only limited usefulness and do not prevent the final deposition of flakes of polymer, after all, on the inner walls of the reactor when several polymerization cycles have gone by. The great number of the above-described methods, however, demonstrates that a universally usable process or a generally applicable regimen for substantially suppressing or for entirely eliminating the formation of polymer deposits on the inner walls of reactors during the suspension polymerization of vinyl chloride does not exist.
The specific processes and measures described in the literature do not have a uniformly positive effect during the manufacture of various types of polyvinyl chloride produced by the suspension polymerization to be used in different applications. The measure of making the polymerization formulations more complicated is unsatisfactory. Treatment of the reactor walls and/or units disposed therein produces the risk that the properties of the product will be adversely altered by the addition and/or application of, for example, dyes. Furthermore, the coating of inner reactor walls is expensive, especially since such coating step must be repeated afer a few batches, and disadvantages are encountered from a safety viewpoint, if the aforementioned substances must be applied within the polymerization vessel as such or dissolved in a small amount of alcohol or benzene.
Therefore, it is an object of this invention to avoid the above-discussed disadvantages of the prior art and make available a polymerization process and apparatus which is equipped so that any desired polymerization recipe can be utilized without limitation. It is another object to provide a polymerization process and apparatus in which the deposit of polymer flakes on the walls of the apparatus is avoided even during a continuous operation. Other objects will be apparent to those skilled in the art.